1. Field of the Invention
The present invention relates to an image forming apparatus for correcting registration deviation.
2. Description of the Related Art
An electrophotography is known as an image recording system used in a color image forming apparatus such as a color printer or a color copying machine. The electrophotography is one which forms a latent image on a photoconductive drum by utilizing a laser beam, and develops it with a charged color material (hereinafter referred to as toner). Recording of an image is carried out by transferring and fixing the image developed using the toner onto a transfer paper.
Recent years, because of image formation speed-up of the electrophotographic color image forming apparatus, increasing are tandem type color image forming apparatuses having the same number of developing machines and photoconductive drums as a number of toner colors, and transferring images of different colors one after another on an image transfer belt, and on a recording medium. In this tandem type color image forming apparatus, it is known that a plurality of factors causing the registration deviation exist, and various solutions are proposed for each of the factors.
One of the factors is nonuniformity or fixing-position deviation of lens of a deflection scanner, and attaching-position deviation of the deflection scanner against the main body of the color image forming apparatus. Because of this position deviation, an inclination and a curvature arise in the scanning line, and a degree of the curvature (hereinafter referred to as a profile) becomes different for every color, which leads to the registration deviation. The profile differs in characteristic thereof in each image forming apparatus, that is, in every recording engine, furthermore in each color.
As a countermeasure method for this registration deviation, there exists a method of measuring magnitude of the inclination and the curvature of the scanning line by using an optical sensor, correcting bitmap image data so that they may be canceled, and forming the corrected image (for example, refer to Japanese Patent Laid-Open No. 2004-170755). In this method, since the correction is carried out electrically by processing the image data, a mechanical adjustment member and an adjustment process at the time of assembly becomes unnecessary. Therefore, it becomes possible to miniaturize magnitude of the color image forming apparatus, and the registration deviation can be coped with at low cost. This electric registration deviation correction is divided into one-pixel-basis correction and less-than-one-pixel-basis-basis correction. The one-pixel-basis correction makes the pixel offset in a subscanning direction by one-pixel-basis depending on the correction amount of the inclination and curvature. In the case of using this method, in the image forming apparatus in which the curvature and inclination are about several 100 to 500 micrometers, and the resolution is 600 dpi, an image memory for several ten lines is needed in order to carryout the correction. In the following descriptions, a position made to be offset is referred to as a change point.
The less-than-one-pixel-basis correction adjusts a tone value of image data by before and after pixels in the subscanning direction, as illustrated in FIG. 23. That is, when it has bent upward by the profile, the image data before correction is corrected to a reverse direction in the subscanning side against the direction which the profile indicates. With such a method, by carrying out the less-than-one-pixel-basis correction, an unnatural level difference at the change point boundary line arisen by the one-pixel-basis correction can be dissolved, and smoothing of the image can be attained.
In the case of carrying out the smoothing processing for the image having progressed to a screen processing immediately before printing, the smoothing is carried out by carrying out pulse width modulation (PWM: Pulse Width Modulation) for the laser beam and switching the laser exposure time gradually in the subscanning direction. For example, in the case of a 0.5-pixel less-than-one-pixel-basis correction, a half exposure will be carried out twice vertically in the subscanning direction. Such interpolation processing is materialized for the first time in the relation that an image density is linear against the exposure time of the PWM, and there are many cases where the same density as one of the exposure for one pixel can not reproduced by two of the exposures for 0.5 pixels even under any kinds of conditions. There also exists one which, also against such problems, in addition to interpolating calculation, devises the way to make lighting-up of the PWM get up right and left, etc., and makes an image centroid shift, and carries out registration correction. (Japanese Patent Laid-Open No. 2001-309139) However, when the linearity of the density against the PWM cannot be held, the image data where it is more preferable to carry out the above-mentioned interpolation processing and the image data where image quality thereof will be impaired if the correction is carried out will exist mixedly.
For example, as for a repeated pattern of the same pattern or a design (hereinafter, referred to as pattern image), and as for a character and a thin line or the like which are able to be rendered with office document creation software, visibility of information thereof is enhanced by carrying out the interpolation processing and smoothing. On the contrary, when the interpolation processing is carried out in the change point of a continuous tone image for which the screen processing has been carried out, there exists a problem that unevenness in density arises only on the change point, and the image quality degrades. As the reason, for example in the case of a line growth screen being used, since a thickness of a line which composes the screen will have changed on the change point due to the interpolation processing, macroscopically it seems that the density has changed. When the interpolation processing is carried out against add-on images such as tint blocks, the effect may be impaired, and therefore, the interpolation processing is not suitable.
As described above, in a situation where it is hard to make linearity of the density against the PWM pulse width come out, it is not appropriate to carry out the interpolation processing for any kinds of images, and a certain decision processing has become needed. However, even in the case of carrying out the decision processing, an erroneous decision may arise, and in the case of carrying out high-speed decision processing, there exist problems in which a circuit becomes complicated or the like. As a result of the decision processing, when the interpolation processing is not carried out for the continuous tone image for which the screen processing has been carried out, the level difference of one pixel which occurs at the change point will be allowed, and the level difference also becomes recognized visually as the degradation depending on a type of the image.
Then, for the image for which the screen processing has been carried out, the correction processing which does not make unevenness in density or the level difference of one pixel which may occur on the change point arise even in a situation where it is hard to make the linearity come out easily, has been required.